Bright annealing



Oct. 17, 1933. lNZEL BRIGHT ANNEALING Filed June 26, 1929 MQQkRkMQQM,4/v/vm 4 we 72/1/ 524 n/RE INVENTOR/ TTORNEY5.

Patented Oct. 17, 1933 BRIGHT ANNEALING Augustus B. Kinzel, Beechhurst,Long Island,

N. Y., assignor to Electra Metallurgical Company, a corporation of WestVirginia Application June 26,1929. Serial No. 373,841

6 Claims.

This invention relates to the heat treatment and the annealing ofmetals, and especially to the method of annealing known as brightannealing in which metals and alloys free from coatings are annealedwithout producing discoloration or oxidation of the metal.

My process is primarily applicable to the annealing of steel and ferrousalloys and the description of the process will be made with reference tothe treatment of such materials, although the process may be used inconnection with the treatment of other metals and alloys.

Heretofore, steel has been bright annealed in atmospheres of purenitrogen, pure hydrogen or pure carbon monoxide. The use of suchatmospheres in connection with annealing has various disadvantages. Thehigh degree of purity of the gas required to produce unoxidized cleansurfaces is objectionable from the standpoint of the cost of the gases.Hydrogen and carbon monoxide are combustible and form explosive mixtureswith air and their use in connection with high temperatures ishazardous. The poisonous nature of carbon monoxide makes its use aserious hazard. The presence of extremely small amounts of oxygen, wateror carbon dioxide in pure nitrogen is known to produce blueing of steelat the temperatures of annealing. Attempts have been made to treat airby passing the air through heated carbon so as to reduce the oxygen andcarbon dioxide to carbon monoxide and to use this gas as an atmosphereforbright annealing. The reduction of carbon dioxide is never complete.This gas contains an amount of carbon dioxide due to the equilibrium Iwhich is sufficient to produce a blueing of the steel at annealingtemperatures.

It is among the objects of my invention to provide methods of brightannealing that avoid toxic, fire and explosion hazards, that do notrequire costly materials and methods of manipulation and that arecapable of producing an unoxidized uncolored product.

I have found that the blueing of steel will not occur when it is heattreated or annealed in an atmosphere of impure nitrogen containing about95% or more of nitrogen and the remainder carbon monoxide with orwithout small amounts of carbon dioxide. (Barbon dioxide may be presentin the gas if the amount of carbon dioxide does not exceed theequilibria, 20022 200 +02 which are produced by heating gases containing5% or less of oxygen orcarbon dioxide at certain temperatures which areabove the annealing temperatures. Due to the preponderance of carbonmonoxide the residue of carbon dioxide in such a reduced gas does notdissociate at annealing temperatures so as to produce blueing 0 of thesteel. The condition may be produced by passing a dry nitrogencontaining gas through a layer of carbon, such as charcoal attemperatures above the predetermined annealing temperatures. Dryuntreated gases are preferred, 5 but absolute freedom from'moisture isnot necessary when the gas is reduced with carbon.

The untreated impure nitrogen may consist of about 95% of nitrogen andthe remainder oxygen, carbon dioxide or either of these gases with 7carbon monoxide. Various suitable sources of untreated gas may be found.For example, the untreated gas may be obtained by oxidizing the oxygenof air mainly to carbon dioxide and then removing the carbon dioxide byappropriate 7 means, so as to produce a nitrogen content of at least95%. Another convenient and inexpensive untreated gas is a by-product ofthe process of making oxygen by the distillation of liquid air. In thisprocess, a residue of nitrogen-containing gas having 5.0% or less ofoxygen is produced.

The minimum difierence between the temperature of deoxidizing the impure95% gas and the temperature of annealing is variable according to thetemperature used for annealing. Lower annealing temperatures require agreater difference between the deoxidizing treatment and annealingtemperatures than do higher annealing temperatures, but a dififerencebetween the deoxidizing treatment and annealing temperatures that isgreater than the minimum may be used. For example, for annealing at 600C., the untreated gas may be deoxidized by heating in the presence ofcharcoal to about 700 C. to obtain the best results, and for annealingat 875 C. or lower the gas may be deoxidized at a temperature of about900 C. It is essential that the gas is always deoxidized at a highertemperature than the temperature to which it is subjected in thepresence of the metal.

The curve C shown in the sole figure of the drawing represents theapproximate relationship between the temperatures of annealing and theminimum temperatures of deoxidizing the gas. The abscissa: of the curveC represent the annealing temperatures and ordinates of the curverepresent deoxidizing temperatures. The point A represents a temperatureof 700 C., the minimum deoxidizing temperature for 95%nitrogencontaining gas of the above described composition when thedeoxidized gas is used as the atmosphere in an annealing box containingmetal that is heat treated at 600 C. The point B represents adeoxidizing temperature of 900 C., the minimum deoxidizing temperatureto be used when annealing at 875 C. From the curve it appears thatsmaller difierences between the deoxidizing and annealing temperaturesmay be used as higher annealing temperatures are used. The curverepresents only the approximate relationship between the annealingtemperature and the minimum temperature of deoxidizing the gas withcharcoal since some departures from the exact relationship representedby the curve may be made depending somewhat upon the materials employed.Regardless of the temperature used in the annealing box, the gas isalways deoxidized at temperatures above 600 C.

Higher deoxidiz ing temperatures than 900 C. will produce smallerproportions of carbon dioxide. Such a gas may be used at annealingtemperatures far below the deoxidizing temperatures if desired. Gaswhich is deoxidized at the lower temperatures does not usually containmore than about 0.50% carbon dioxide. When the gas is deoxidized underthe most favorable conditions, no carbon dioxide can be detected by theordinary qualitative tests in small portions of the gas. The use ofatmospheres consisting of at least 95% nitrogen and the remainder carbonmonoxide is contemplated.

As illustrative of my invention, impure nitrogen consisting of about95.5% nitrogen and oxygen is passed through a bed of charcoal which isheated to 900 C. so as to reduce the oxygen and produce a gas which issubstantially in equilibrium with carbon at this temperature. The air ina carefully luted annealing box which contains bright unoxidized sheetsof steel, is displaced by the deoxidized gas. The sheets are thenannealed at 800 C. in the presence of the deoxidized gas. If heateddeoxidized gas is used, care should be taken to avoid heating the metalabove the annealing temperatures by the heated gas. A slow current ofdeoxidized gas may be passed into the annealing box during annealing andduring the cooling of the annealing box to prevent influx of air.

The amount of carbon monoxide in the above described atmospheres isinsufficient to support combustion or to form explosive mixtures withair. The liability of carbon monoxide poisoning from leakage of this gasis practically eliminated by reason of its low content of carbonmonoxide and the diffusion of the discharged gas in the atmosphere.

While I have disclosed the use of nitrogen, which I prefer to usebecause of cheapness and availability, I. may if I so desire, replacethe nitrogen, in whole or in part, with other essentially inert gasessuch, for example, as helium or argon.

I claim:

'1. The method of bright-annealing metal articles which comprisesreducing a substantially dry gaseous atmosphere containing at least 95%nitrogen and the remainder oxygen containing by heating the atmospherein the presence temperatures than the pregas of carbon to higherdetermined temperatures of annealing, and

heating the articles to said predetermined temperatures in the reducedatmosphere.

2. The method of bright-annealing metal articles which comprisesreducing a gas consisting of at least 95% nitrogen and the remainderoxygen, by heating the gas in the presence of carbon to highertemperatures than the predetermined temperatures of annealing, and heattreating the articles at said predetermined temperatures in anatmosphere consisting of said reduced gas.

3. The method of bright-annealing metal articles which comprisesreducing a gas containing at least 95% nitrogen and the remainder anamount of oxygen containing gas capable of discoloring said metal atpredetermined annealing temperatures, by heating said gas above thepredetermined temperatures in the presence of carbon, and heat treatingthe metals at said predetermined temperatures in an atmosphereconsisting of said reduced gas.

4. The method of heat-treating metals which comprises the production ofan atmosphere having substantially more carbon monoxide than required bythe equilibrium, 2002 ZCO+O2, which may be produced by deoxidizing anitrogen containing gas comprising at least 95.0% nitrogen and theremainder one or more than one of the gases of said equilibrium byheating the gas in the presence of carbon to predetermined temperatureshigher than the heat treating ternperatures, and heat treating the metalin the presence of said atmosphere at the heat treating temperatures.

5. The method of annealing metals which comprises the production of anatmosphere having substantially more carbon monoxide than theequilibrium, 2CO2.; .2CO+O2, which may be produced by deoxidizing animpure nitrogen containing at least 95.0% of nitrogen and the remainderone or more than one of the gases of said equilibrium by heating theimpure nitrogen in the presence of carbon to deoxidizing temperaturesabove 600 C. said deoxidizing temperatures bearing about the relation toannealing temperatures as those represented by a smooth curve having asrectangular coordinates deoxidizing temperatures and annealingtemperatures, the position of the curve on the coordinates being 12'determined by the following deoxidizing temperatures: 700 C., 850 C.,1000 C., 1150 C., and by the following corresponding annealingtemperatures: 600 C., 810 C., 990 C., 1140 C., respectively, and heatingthe metal in the presence of 12 said atmosphere to temperatures which donot exceed said corresponding annealing temperatures.

6. The method of bright-annealing metals which comprises producing anatmosphere hav- 12 ing more carbon monoxide than is required for theequilibrium, 2002:32004-02, so that the atmosphere will not discoloriron-containing alloys at annealing temperatures, and heating the metalin said atmosphere to said annealing tempera- 13 tures; the productionof said atmosphere being effected by a method which comprisesdeoxidizing a gas containing at least 95.0% nitrogen, the remainderbeing one or -more of the gases of said equilibrium, by heating the gasin the pres- 1. ence of carbon to a deoxidizing temperature at least ashigh as that represented by the ordinate at the intersection of theabscissarepresenting the annealing temperature with a smooth curvehaving rectangular coordinates, the curve being 1 determined by thefollowing deoxidizing temperatures as ordinates: 700" C., 350 C., 1000C., 1159 C., and the following annealing temperatures as abscissa: 600C., 810 C., 990 C., 1140" C.

AUGUSTUS B. KINZEL. 1

